1. Field of the Invention
The invention relates to a method of fabricating a silicon-on-insulator substrate including an insulator and a silicon active layer formed on the insulator, and more particularly to such a method including a hydrogen ion separation process. The invention also relates to a silicon-on-insulator substrate suitable for a hydrogen ion separation process.
2. Description of the Related Art
A silicon-on-insulator (hereinafter, referred to as xe2x80x9cSOIxe2x80x9d) structure including a silicon active layer formed on an insulator is considered promising as a substrate to be used for next generation LSI. There have been suggested various methods of fabricating an SOI substrate.
One of such various methods is a method having the steps of forming an oxide film on a surface of a silicon substrate, implanting hydrogen ions into the silicon substrate, overlapping the silicon substrate to a support substrate, and applying heat treatment to the thus overlapped silicon and support substrates to thereby separate the silicon substrate into two pieces at a region to which hydrogen ions have been implanted (hereinafter, this method is referred to as xe2x80x9chydrogen ion separation processxe2x80x9d).
Hereinbelow is explained the above-mentioned hydrogen ion separation process with reference to FIGS. 1A to 1F.
First, as illustrated in FIG. 1A, there are prepared a silicon substrate 1 and a support substrate 2. A silicon wafer having (100) plane or a plane slightly inclined relative to (100) plane as a principal plane is usually selected as the silicon substrate 1. The same silicon wafer as just mentioned is usually also selected as the support substrate 2.
Then, as illustrated in FIG. 1B, a silicon dioxide film 3 is formed at a surface of the silicon substrate 1. This silicon dioxide film 3 will make an insulating film in an SOI structure. Hence, the silicon dioxide film 3 is designed to have a thickness equal to a thickness of a buried oxide film required for fabrication of a device on an SOI substrate.
Then, as illustrated in FIG. 1C, hydrogen is ion-implanted into the silicon substrate 1 through the silicon dioxide film 3. The thus ion-implanted hydrogen 4 stays in the silicon substrate 1 at a certain depth. When the silicon substrate 1 is subject to heat treatment in a later step, the silicon substrate 1 is separated at that depth into two pieces. One of the two pieces to which the silicon dioxide film 3 belongs makes an SOI active layer in an SOI structure. Hence, in this step of ion-implanting hydrogen into the silicon substrate 1, acceleration energy is controlled for the SOI active layer to have a desired thickness. The silicon substrate 1 is usually implanted at about 30-200 KeV with doses of 1xc3x971016-3xc3x971017H+ cmxe2x88x922. The implanted hydrogen ions break bondings between silicon atoms in silicon crystal, and terminate non-bonded hands of silicon atoms.
Then, as illustrated in FIG. 1D, the silicon substrate 1 is laid on top of the support substrate 2 so that surfaces of them make direct contact with each other. Thereafter, the thus overlapped silicon substrate 1 and support substrate 2 are subject to heat treatment.
The heat treatment has two stages.
In a first stage, heat treatment to be carried out at a relatively low temperature in the range of 300 to 800 degrees centigrade is applied to the overlapped silicon substrate 1 and support substrate 2. By carrying out the first stage, the silicon substrate 1 and the support substrate 2 make close contact with each other, and at the same time, the silicon substrate 1 is separated into two pieces at the depth at which hydrogen 4 have been ion-implanted, as illustrated in FIG. 1E.
Hydrogen 4 having been ion-implanted into the silicon substrate 1 in the step illustrated in FIG. 1C is agglomerated at (111) plane or at (100) plane which is parallel to a surface of the silicon substrate 1, as a temperature raises in the first stage of the heat treatment, to thereby form cavities in the silicon substrate 1. If the support substrate 2 is not laid on the silicon substrate 1, a surface layer of the silicon substrate 1 would be peeled off by pressure of hydrogen gas generated in the first stage heat treatment carried out at 300-800 degrees centigrade.
However, in accordance with the hydrogen ion separation process, since the support substrate 2 makes close contact with the silicon substrate 1 with the silicon dioxide film 3 being sandwiched therebetween, the silicon substrate 1 is separated into two pieces one of which remains non-separated from the silicon dioxide film 3 and the support substrate 2. One of the two pieces, which remains on the silicon dioxide film 3, acts as an SOI active layer 5. Thus, there is formed an SOI structure including the support substrate 2, the silicon dioxide film 3 located on the support substrate 2, and the SOI active layer 5 formed on the silicon dioxide film 3. As mentioned above, the SOI active layer 5 is one of the two pieces of the silicon substrate 1.
The separation of the silicon substrate 1 into two pieces is considered partially because of force of deformation caused due to a difference in a thermal expansion coefficient between the support substrate 2 and the silicon dioxide film 3.
Then, in a second stage of the heat treatment, the SOI structure including the SOI active layer 5, the silicon dioxide film 3, and the support substrate 2 is subject to heat treatment at a relatively high temperature, specifically, at 1000 degrees centigrade or greater. Thus, as illustrated in FIG. 1F, there is completed an SOI substrate.
The second stage heat treatment is carried out for the purpose of enhancing bonding force between the support substrate 2 and the silicon dioxide film 3, because it would be impossible to ensure sufficient bonding force therebetween only by the first stage heat treatment.
In the specification, a silicon wafer is distinctive from a silicon substrate. Specifically, the term xe2x80x9csilicon waferxe2x80x9d is used as a generic name for indicating a wafer manufactured by CZ process, for instance, whereas the term xe2x80x9csilicon substratexe2x80x9d is used to indicate a substrate on which an active layer is to be formed in fabrication of an SOI substrate.
When fabrication of an SOI substrate by hydrogen ion separation process is repeated, the other of the two pieces of the silicon substrate 1, removed away in the above-mentioned first stage heat treatment, may be re-used as the silicon substrate 1 or as the support substrate 2 in next fabrication of an SOI substrate.
For instance, Japanese Unexamined Patent Publications Nos. 2-46770 and 9-22993 have suggested fabrication of an SOI substrate by such a hydrogen ion separation process as mentioned above.
Apart from those Publications, fabrication of an SOI substrate by a hydrogen ion separation process has been reported in (a) C. Maleville et al., Silicon-on-Insulator and Devices VII, pp. 34, Electrochem. Soc., Pennington, 1996, and (b) Abe et al., Applied Physics, Vol. 66, No. 11, pp. 1220, 1997.
The hydrogen ion separation process for fabrication of an SOI substrate, having been explained so far, has many advantages as follows, for instance, in comparison with other processes for fabrication of an SOI substrate.
First, it is possible to control a thickness of an SOI active layer, since a thickness of an SOI active layer is dependent on a range distance of ion-implanted hydrogen. The hydrogen ion separation process is suitable in particular for fabrication of a super-thin film SOI substrate.
Second, it is possible to uniformize a thickness of an SOI active layer, and an SOI substrate can readily have a large diameter.
Third, it is possible to reduce fabrication cost, because the hydrogen ion separation process is comprised of steps of ion-implantation and heat treatment both of which are compatible with an ordinary LSI fabrication process.
Fourth, great designability is ensured for thicknesses of an SOI active layer and a buried oxide film.
Fifth, an efficiency for using a wafer is higher than that of other SOI substrate fabrication processes. Specifically, an SOI substrate fabrication process such as a process including the steps of laying a first substrate on a second substrate, and polishing the first or second substrate to thereby make a thin film requires preparation of two wafers for fabrication of an SOI substrate. On the other hand, the hydrogen ion separation process requires preparation of only one wafer for fabrication of an SOI substrate by using a silicon wafer as a support substrate, and re-using a removed piece of a silicon substrate as a silicon or support substrate in next fabrication of an SOI substrate.
However, the hydrogen ion separation process is accompanied with a problem that a resultant SOI active layer includes a lot of crystal defects therein. An SOI active layer made in accordance with the conventional hydrogen ion separation process usually includes crystal defects by the number of about 1xc3x97103 to 1xc3x97104/cm2. Such a lot of crystal defects exert harmful influence on characteristics of a device to be formed on an SOI active layer including the crystal defects.
From the standpoint of practical use, it is absolutely necessary to reduce a crystal defect density in an SOI active layer down to about 1xc3x9710/cm2 or smaller in order to use an SOI substrate in next generation LSI.
However, not only a method of reducing crystal defects, but also the reason why crystal defects are generated have not been found so far.
In view of the above-mentioned problem, it is an object of the present invention to make it possible to reduce a crystal defect density in an SOI active layer in fabrication of an SOI substrate by means of a hydrogen ion separation process.
In one aspect of the present invention, there is provided a method of fabricating a silicon-on-insulator substrate, including the steps of (a) forming a silicon substrate at a surface thereof with an oxygen-containing region containing oxygen at such a concentration that oxygen is not precipitated in the oxygen-containing region in later mentioned heat treatment, (b) forming a silicon oxide film at a surface of the silicon substrate, (c) implanting hydrogen ions into the silicon substrate through the silicon oxide film, (d) overlapping the silicon substrate and a support substrate each other so that the silicon oxide film makes contact with the support substrate, and (e) applying heat treatment to the thus overlapped silicon substrate and support substrate to thereby separate the silicon substrate into two pieces at a region into which the hydrogen ions have been implanted, one of the two pieces remaining on the silicon oxide film as a silicon-on-insulator active layer, the support substrate, the silicon oxide film located on the support substrate, and the silicon-on-insulator active layer formed on the silicon oxide film defining a silicon-on-insulator structure.
It is preferable that the heat treatment in the step (e) is comprised of first heat treatment to be carried out at a temperature in the range of 300 to 800 degrees centigrade both inclusive, and second heat treatment to be carried out at a temperature in the range of 1000 to 1200 degrees centigrade.
It is preferable that the oxygen-containing region is designed to contain oxygen at a concentration equal to or smaller than 1xc3x971018/cm3.
In the step (c), ions other than hydrogen ions may be also implanted into the silicon substrate together with the hydrogen ions through the silicon oxide film.
It is preferable that the method further includes the step (f) of forming an oxide film at a surface of the support substrate, the silicon substrate and the support substrate being overlapped each other in the step (d) so that the silicon oxide films of the silicon substrate and the support substrate make contact with each other.
It is preferable that the method further includes the step (g) of causing at least one of the silicon substrate and the support substrate to absorb hydroxyl group thereinto prior to carrying out the step (d).
For instance, the step (g) may be comprised of the steps of removing a natural oxide film out of a surface of the substrate(s), and rinsing the substrate(s) in super-pure water.
It is preferable that the method further includes the step (h) of selecting a material of which the support substrate is composed, among one of silicon, quartz glass, sapphire, SiC, and diamond.
It is preferable that the support substrate and the silicon substrate are designed to have common characteristics, and the method further includes the step of (i) using the other of the two pieces of the silicon substrate as a support substrate in next fabrication of a silicon-on-insulator substrate.
It is preferable that the method further includes the step of (i) using the other of the two pieces of the silicon substrate as a silicon substrate in next fabrication of a silicon-on-insulator substrate.
There is further provided a method of fabricating a silicon-on-insulator substrate, including the steps of (a) forming a silicon oxide film at a surface of a silicon substrate containing oxygen at such a concentration that oxygen is not precipitated in the silicon substrate in later mentioned heat treatment, (b) implanting hydrogen ions into the silicon substrate through the silicon oxide film, (c) overlapping the silicon substrate and a support substrate each other so that the silicon oxide film makes contact with the support substrate, and (d) applying heat treatment to the thus overlapped silicon substrate and support substrate to thereby separate the silicon substrate into two pieces at a region into which the hydrogen ions have been implanted, one of the two pieces remaining on the silicon oxide film as a silicon-on-insulator active layer, the support substrate, the silicon oxide film located on the support substrate, and the silicon-on-insulator active layer formed on the silicon oxide film defining a silicon-on-insulator structure.
It is preferable that the method further includes the step of making the silicon substrate by FZ process or MCZ process.
There is still further provided a method of fabricating a silicon-on-insulator substrate, including the steps of (a) forming a silicon substrate at a surface thereof with an oxygen-containing region containing oxygen at a lower concentration than a concentration of other regions of the silicon substrate so that oxygen is not precipitated in the oxygen-containing region in later mentioned heat treatment, (b) forming a silicon oxide film at a surface of the silicon substrate, (c) implanting hydrogen ions into the silicon substrate through the silicon oxide film, (d) overlapping the silicon substrate and a support substrate each other so that the silicon oxide film makes contact with the support substrate, and (e) applying heat treatment to the thus overlapped silicon substrate and support substrate to thereby separate the silicon substrate into two pieces at a region into which the hydrogen ions have been implanted, one of the two pieces remaining on the silicon oxide film as a silicon-on-insulator active layer, the support substrate, the silicon oxide film located on the support substrate, and the silicon-on-insulator active layer formed on the silicon oxide film defining a silicon-on-insulator structure.
It is preferable that the step (a) is carried out by applying heat treatment to the silicon substrate at 1000 degrees centigrade or greater in atmosphere containing oxygen at 1% or smaller, in which case, it is preferable that the heat treatment is carried out at 1300 degrees centigrade or smaller.
In another aspect of the present invention, there is provided a silicon-on-insulator substrate including (a) a substrate, (b) an insulating film formed on the substrate, and (c) a silicon layer containing oxygen at a concentration equal to or smaller than 1xc3x971018/cm3.
It is preferable that the substrate contains hydroxyl group therein.
The substrate may be composed of one of silicon, quartz glass, sapphire, SiC, and diamond.
Hereinbelow is explained the principle of the present invention by which it is possible to fabricate an SOI substrate having crystal defects by the smaller number than crystal defects of an SOI substrate fabricated in accordance with a conventional hydrogen ion separation process.
The inventor inspected the reason why crystal defects are generated, and found out the following.
First, it was found out that a silicon substrate used in a conventional SOI substrate contained oxygen in a considerable amount, specifically, at a concentration of 1xc3x971018/cm3 or greater. In fabrication of an SOI substrate from such a silicon substrate by a hydrogen ion separation process, two stages heat treatment to be carried out after overlapping a silicon substrate and a support substrate each other generates precipitation cores of oxygen contained in a silicon substrate, and facilitates precipitation of oxygen. As a result, oxygen originally contained in a silicon substrate is precipitated, and the thus precipitated oxygen causes deformation in a silicon substrate, which in turn causes dislocation and/or rod-shaped crystal defects. In conclusion, crystal defects in an SOI active layer fabricated by a hydrogen ion separation process are caused by oxygen originally contained in a silicon substrate.
It was further found out that if an SOI substrate was made from a silicon substrate containing oxygen at a relatively low concentration, even after the above-mentioned two stage heat treatment was applied, oxygen was suppressed from being precipitated, and as a result, it was possible to prevent generation of crystal defects in an SOI active layer.
A crystal defect density of an SOI active layer is not in proportion to an oxygen concentration of a silicon substrate. It was found out, that if a silicon substrate contained oxygen at a critical concentration or smaller, crystal defects were no longer generated. It was also found out that such a critical oxygen concentration was dependent on conditions of heat treatment to be carried out in an SOI substrate fabrication process, and was about 1xc3x971018/cm3 in ordinary conditions of heat treatment.
The above-mentioned phenomenon is considered that a silicon substrate has to contain oxygen at a critical concentration or greater in order that oxygen precipitation cores are generated during heat treatment, and that such a critical oxygen concentration is equal to about 1xc3x971018/cm3.
Based on the above-mentioned discovery of the inventor, it is understood that it would be possible to prevent generation of crystal defects and thereby fabricate a qualified SOI substrate having less crystal defects by using a silicon substrate having a surface layer which will make an SOI active layer and which contains oxygen at a concentration of about 1xc3x971018/cm3 or smaller.
In the present invention, there may be used a silicon substrate containing oxygen at a relatively low concentration. As an alternative, there may be used a silicon substrate containing oxygen at a relatively high concentration, after the silicon substrate is treated to reduce an oxygen concentration at a surface thereof For instance, a silicon substrate containing oxygen at a relatively low concentration, fabricated by FZ process or MCZ process, may be used without any pre-treatment. A silicon substrate containing oxygen at a relatively high concentration, fabricated by CZ process, may be used after an oxygen concentration at a surface thereof is reduced, for instance, by applying heat treatment in argon atmosphere thereto.
In order to strengthen a bonding force between a silicon oxide film to be formed on a surface of a silicon substrate, and a support substrate, a silicon substrate and/or a support substrate may be caused to absorb hydroxyl group (OH group) thereinto before overlapping them each other.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.